Normally if no value has been saved for a custom field then the key would not exist for it, which is what the code I offered was testing for.
But I looked in the database and all of your posts have the key present, even when there is no value (I suspect it is something particular to the image field which I'd not noticed before), and so I changed the test to see if the image field exists but is not an empty string.
That now works.
Sorry it has taken a long time to arrive at the solution. The original problem with the wpv-conditional shortcode arises from a technical limitation in PHP about the length of the content inside the conditional shortcode, otherwise it would have been a perfectly valid solution.